1. Field of the Invention
The present invention relates to a power unit for a motorcycle. In the power unit, a continuously variable transmission is housed in a transmission chamber of an engine body. The continuously variable transmission includes a drive pulley provided on a drive pulley shaft, to which the torque is transmitted from a crankshaft, a driven pulley provided on the driven pulley shaft with its axis being parallel to the drive pulley shaft and a belt looped between the drive pulley and the driven pulley. Stepless changes in the transmission of power from the drive pulley shaft to the driven pulley shaft are accomplished by changing the effective diameter with which the belt actually wraps around each of the drive and the driven pulleys.
2. Description of Background Art
Japanese Patent Application Laid-Open Publication No. 2000-73724 discloses a power unit for a motorcycle equipped with a starter clutch that becomes in Clutch-On state once the revolution of the torque transmitted from the crankshaft to the drive pulley shaft of the continuously variable transmission exceeds a preset value. In a power unit for a four-wheel vehicle disclosed in Japanese Unexamined Patent Application Publication No. SHO63-103784, a plurality of clutches and a continuously variable transmission are housed in a transmission chamber. The switching between engagement and disengagement of each of the clutches is hydraulically controlled. In addition, the speed change performed by the continuously variable transmission is also hydraulically controlled.
The power unit as disclosed in Japanese Patent Application Laid-Open Publication No. 2000-73724 includes no other clutches other than the starter clutch. In this power unit with a configuration, for example, in which the starter clutch is provided on the drive pulley shaft, the load on the starter system can be reduced. When the vehicle is stopped, the starter clutch in the power unit is sometimes disengaged before the drive pulley and the driven pulley in the continuously variable transmission are allowed to have respective pulley groove widths corresponding to a low-speed state. In this case, next time the vehicle starts, the vehicle has to start with its continuously variable transmission being in a high-speed state.
On the other hand, the power unit disclosed in Japanese Unexamined Patent Application Publication No. SHO63-103784 includes a plurality of clutches, and the switching between the engagement and disengagement of each clutch is hydraulically controlled. The larger the number of oil passages becomes, the more complex the configuration of oil passages becomes. Consequently, the formation of oil passages needs special consideration.